HTTP header fields are a list of strings sent and received by both the client program and server on every HTTP request and response. These headers are usually invisible to the end-user and are only processed or logged by the server and client applications. They define how information sent/received through the connection are encoded (as in Content-Encoding), the session verification and identification of the client (as in browser cookies, IP address, user-agent) or their anonymity thereof (VPN or proxy masking, user-agent spoofing), how the server should handle data (as in Do-Not-Track or Global Privacy Control), the age (the time it has resided in a shared cache) of the document being downloaded, amongst others.
General format
In HTTP version 1.x, header fields are transmitted after the request line (in case of a request HTTP message) or the response line (in case of a response HTTP message), which is the first line of a message. Header fields are colon-separated key-value pairs in clear-text string format, terminated by a carriage return (CR) and line feed (LF) character sequence. The end of the header section is indicated by an empty field line, resulting in the transmission of two consecutive CR-LF pairs. In the past, long lines could be folded into multiple lines; continuation lines are indicated by the presence of a space (SP) or horizontal tab (HT) as the first character on the next line. This folding was deprecated in RFC 7230.[1]
HTTP/2[2] and HTTP/3 instead use a binary protocol, where headers are encoded in a single HEADERS and zero or more CONTINUATION frames using HPACK[3] (HTTP/2) or QPACK (HTTP/3), which both provide efficient header compression. The request or response line from HTTP/1 has also been replaced by several pseudo-header fields, each beginning with a colon (:).
Header field names are case-insensitive.[4] This is in contrast to HTTP method names (GET, POST, etc.), which are case-sensitive.[5]
HTTP/2 makes some restrictions on specific header fields (see below).
Non-standard header fields were conventionally marked by prefixing the field name with X- but this convention was deprecated in June 2012 because of the inconveniences it caused when non-standard fields became standard.[6] An earlier restriction on use of Downgraded- was lifted in March 2013.[7]
Field values
A few fields can contain comments (i.e. in User-Agent, Server, Via fields), which can be ignored by software.[8]
Many field values may contain a quality (q) key-value pair separated by equals sign, specifying a weight to use in content negotiation.[9] For example, a browser may indicate that it accepts information in German or English, with German as preferred by setting the q value for de higher than that of en, as follows:
Accept-Language: de; q=1.0, en; q=0.5
Size limits
The standard imposes no limits to the size of each header field name or value, or to the number of fields. However, most servers, clients, and proxy software impose some limits for practical and security reasons. For example, the Apache 2.3 server by default limits the size of each field to 8,190 bytes, and there can be at most 100 header fields in a single request.[10]
Request fields
Standard request fields
Name
Description
Example
Status
Standard
A-IM
Acceptable instance-manipulations for the request.[11]
The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested.
Mandatory since HTTP/1.1.[17]
If the request is generated directly in HTTP/2, it should not be used.[18]
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Settings header field. The HTTP2-Settings header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.[19][20]
Only perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it.
This is the address of the previous web page from which a link to the currently requested page was followed. (The word "referrer" has been misspelled in the RFC as well as in most implementations to the point that it has become standard usage and is considered correct terminology.)
The transfer encodings the user agent is willing to accept: the same values as for the response header field Transfer-Encoding can be used, plus the "trailers" value (related to the "chunked" transfer method) to notify the server it expects to receive additional fields in the trailer after the last, zero-sized, chunk.
The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer coding.
Tells a server which (presumably in the middle of a HTTP -> HTTPS migration) hosts mixed content that the client would prefer redirection to HTTPS and can handle Content-Security-Policy: upgrade-insecure-requests
Upgrade-Insecure-Requests: 1
X-Requested-With
Mainly used to identify Ajax requests (most JavaScript frameworks send this field with value of XMLHttpRequest); also identifies Android apps using WebView[23]
Requests a web application to disable their tracking of a user. This is Mozilla's version of the X-Do-Not-Track header field (since Firefox 4.0 Beta 11). Safari and IE9 also have support for this field.[25] On March 7, 2011, a draft proposal was submitted to IETF.[26] The W3C Tracking Protection Working Group is producing a specification.[27]
A de facto standard for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer. Superseded by Forwarded header.
A de facto standard for identifying the original host requested by the client in the Host HTTP request header, since the host name and/or port of the reverse proxy (load balancer) may differ from the origin server handling the request. Superseded by Forwarded header.
A de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (or a load balancer) may communicate with a web server using HTTP even if the request to the reverse proxy is HTTPS. An alternative form of the header (X-ProxyUser-Ip) is used by Google clients talking to Google servers. Superseded by Forwarded header.
Requests a web application to override the method specified in the request (typically POST) with the method given in the header field (typically PUT or DELETE). This can be used when a user agent or firewall prevents PUT or DELETE methods from being sent directly (this is either a bug in the software component, which ought to be fixed, or an intentional configuration, in which case bypassing it may be the wrong thing to do).
Links to an XML file on the Internet with a full description and details about the device currently connecting. In the example to the right is an XML file for an AT&T Samsung Galaxy S2.
Implemented as a misunderstanding of the HTTP specifications. Common because of mistakes in implementations of early HTTP versions. Has exactly the same functionality as standard Connection field.
The Save-Data client hint request header available in Chrome, Opera, and Yandex browsers lets developers deliver lighter, faster applications to users who opt-in to data saving mode in their browser.
The Sec-GPC (Global Privacy Control) request header indicates whether the user consents to a website or service selling or sharing their personal information with third parties.
A server uses "Alt-Svc" header (meaning Alternative Services) to indicate that its resources can also be accessed at a different network location (host or port) or using a different protocol
When using HTTP/2, servers should instead send an ALTSVC frame.[50]
An opportunity to raise a "File Download" dialogue box for a known MIME type with binary format or suggest a filename for dynamic content. Quotes are necessary with special characters.
This field is supposed to set P3P policy, in the form of P3P:CP="your_compact_policy". However, P3P did not take off,[54] most browsers have never fully implemented it, a lot of websites set this field with fake policy text, that was enough to fool browsers the existence of P3P policy and grant permissions for third party cookies.
P3P: CP="This is not a P3P policy! See https://en.wikipedia.org/wiki/Special:CentralAutoLogin/P3P for more info."
Permanent
Pragma
Implementation-specific fields that may have various effects anywhere along the request-response chain.
If an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date.[56]
Example 1: Retry-After: 120
Example 2: Retry-After: Fri, 07 Nov 2014 23:59:59 GMT
The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer coding.
Tracking Status header, value suggested to be sent in response to a DNT(do-not-track), possible values:
"!" — under construction
"?" — dynamic
"G" — gateway to multiple parties
"N" — not tracking
"T" — tracking
"C" — tracking with consent
"P" — tracking only if consented
"D" — disregarding DNT
"U" — updated
Tells downstream proxies how to match future request headers to decide whether the cached response can be used rather than requesting a fresh one from the origin server.
Clickjacking protection: deny - no rendering within a frame, sameorigin - no rendering if origin mismatch, allow-from - allow from specified location, allowall - non-standard, allow from any location
Tells the browser to refresh the page or redirect to a different URL, after a given number of seconds (0 meaning immediately); or when a new resource has been created[clarification needed]. Header introduced by Netscape in 1995 and became a de facto standard supported by most web browsers. Eventually standardized in the HTML Living Standard in 2017.[64]
CGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 9110.[66]
Status: 200 OK
Timing-Allow-Origin
The Timing-Allow-Origin response header specifies origins that are allowed to see values of attributes retrieved via features of the Resource Timing API, which would otherwise be reported as zero due to cross-origin restrictions.[67]
Provide the duration of the audio or video in seconds. Not supported by current browsers – the header was only supported by Gecko browsers, from which support was removed in 2015.[69]
The only defined value, "nosniff", prevents Internet Explorer from MIME-sniffing a response away from the declared content-type. This also applies to Google Chrome, when downloading extensions.[71]
Specifies the technology (e.g. ASP.NET, PHP, JBoss) supporting the web application (version details are often in X-Runtime, X-Version, or X-AspNet-Version)
Recommends the preferred rendering engine (often a backward-compatibility mode) to use to display the content. Also used to activate Chrome Frame in Internet Explorer. In HTML Standard, only the IE=edge value is defined.[75]
If a web server responds with Cache-Control: no-cache then a web browser or other caching system (intermediate proxies) must not use the response to satisfy subsequent requests without first checking with the originating server (this process is called validation). This header field is part of HTTP version 1.1, and is ignored by some caches and browsers. It may be simulated by setting the Expires HTTP version 1.0 header field value to a time earlier than the response time. Notice that no-cache is not instructing the browser or proxies about whether or not to cache the content. It just tells the browser and proxies to validate the cache content with the server before using it (this is done by using If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match attributes mentioned above). Sending a no-cache value thus instructs a browser or proxy to not use the cache contents merely based on "freshness criteria" of the cache content. Another common way to prevent old content from being shown to the user without validation is Cache-Control: max-age=0. This instructs the user agent that the content is stale and should be validated before use.
The header field Cache-Control: no-store is intended to instruct a browser application to make a best effort not to write it to disk (i.e not to cache it).
The request that a resource should not be cached is no guarantee that it will not be written to disk. In particular, the HTTP/1.1 definition draws a distinction between history stores and caches. If the user navigates back to a previous page a browser may still show you a page that has been stored on disk in the history store. This is correct behavior according to the specification. Many user agents show different behavior in loading pages from the history store or cache depending on whether the protocol is HTTP or HTTPS.
The Cache-Control: no-cache HTTP/1.1 header field is also intended for use in requests made by the client. It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource. The Pragma: no-cache header field, defined in the HTTP/1.0 spec, has the same purpose. It, however, is only defined for the request header. Its meaning in a response header is not specified.[77] The behavior of Pragma: no-cache in a response is implementation specific. While some user agents do pay attention to this field in responses,[78] the HTTP/1.1 RFC specifically warns against relying on this behavior.
^"HTML Living Standard 4.2.5.3 Pragma directives, X-UA-Compatible state". WHATWG. March 12, 2021. Retrieved March 14, 2021. For meta elements with an http-equiv attribute in the X-UA-Compatible state, the content attribute must have a value that is an ASCII case-insensitive match for the string"IE=edge".
